762 



THE POPULAR SCIENCE MONTHLY 



Bystem. The city of Gratz, containing 

 80,000 inhabitants, has this system in use 

 in every house, and has thus demonstrated 

 the practicability of using it in large cities. 

 As an illustration of the profit to be derived 

 from human excrement when fairly tested 

 as a fertilizer, Mr. Lepmann refers to the 

 fact that, between the years 1850 and 1864, 

 the price of that obtained from the barracks 

 increased forty-five fold. 



Mental Exertions governed y Law. 

 Prof. Heinrichs read a paper at the Du- 

 buque scientific meeting, " On the Law of 

 Probability as applied to the Determination 

 of Mental Exertions." The following is a 

 summary : 



All phenomena in the physical world, 

 exhibited by individuals of a mass subject 

 to certain given influences, are regulated by 

 the so-called law of probability. This has 

 long been practically used by the various 

 insurance companies, which employ millions 

 of dollars a year ; however uncertain the 

 health of any given individual, the number 

 of individuals dying each year in a mass of 

 a hundred thousand individuals is percep- 

 tibly constant. So also the height of the 

 stature of the individual in a greatly-vary- 

 ing quantity ; but the number of individu- 

 als in an army having a certain definite 

 height is very nearly constant, and deter- 

 mined by the law of probability. The ap- 

 plication of this law of probability to the 

 affairs of the individual man may be 

 studied in the works of Quetelet. By 

 several of our modern chemists the same 

 law has been applied to the various chemi- 

 cal processes. If the laws which regulate 

 mental work and mental phenomena are not 

 radically different from those which we 

 study in the physical world proper, then the 

 law of probability ought to be equally ap- 

 plicable to the mental stature of man, as 

 we long ago have found it to be applicable 

 to the bodily stature of the same. By 

 very careful determination of the relative 

 grade of the individuals composing the 

 large classes which have been instructed in 

 the elements of physics at the State Uni- 

 versity of Iowa, the author has, during the 

 past three years, had abundant means to 

 test the applicability of the law of proba- 

 bility to mental exertions. The student's 



standing is determined by adding the nu- 

 merical values of his credit for oral exami- 

 nation on the subject studied to the grade 

 expressing his daily recitation and his prac- 

 tical work in the laboratory. Since these 

 three quantities are determined independent- 

 ly of one another, and often by different per- 

 sons (the class being instructed by the pro- 

 fessor and two assistants), we have some 

 guarantee against the accumulation of per- 

 sonal errors in this determination. Thus, 

 in a class of sixty-seven students in the 

 elements of physics, the following table 

 shows the observed number of students per 

 hundred who have obtained the standing 

 given, also the calculated number of stu- 

 dents who, according to the law of proba- 

 bility, should have obtained the same de- 

 gree. It will be seen that the two numbers 

 agree very closely : 



NUMBER OF STUDENTS IN 100. 



Dr. Hooker and the Eew Gardens. 



The English papers have latterly had much 

 to say of the difficulty between Dr. Hooker, 

 Superintendent of the Kew Botanical Gar- 

 dens, and Mr. Ayrton, a member of the 

 Government, and Superintendent of Public 

 Works. The Kew Gardens are part of an 

 old royal park situated a few miles out 

 of London, and have been developed to 

 their present great extent and remarkable 

 beauty, as well as in their scientific richness, 

 mainly through the labors of the celebrated 

 botanist Sir William Hooker, and of his 

 son, Dr. Joseph Dalton Hooker, the present 

 director. Without the genius, learning, en- 

 thusiasm, and, it may be added, the liber- 

 al pecuniary aid of .these gentlemen, the 

 Gardens would probably never have been 

 created. They have been called into ex- 

 istence mainly through their agencies, and 

 are now the pride of the nation, and are 



